This adds three new/modified features to the diff section of review request emails:

The diff section is now omitted for review requests which do not actually contain diffs.  This has the effect of also preventing the View Diff link, which for the no diffs case was previously a link to a page that did not exist.
Update View Diff link to point to the specific version of the diff that is relevant at the time that the email is produced, rather than always referencing the most recent version of the diff.  The benefit here is that the link will take you to the diff version that is pertinent relative to the email, even if subsequent diffs have been posted.
Add Show Changes link to provide a shortcut to the relevant interdiff.  This mimics the links available in a "Review request changed" element in the main review request UI when an updated diff is posted.  Correspondingly, the link is included in emails for all requests where an updated diff has been posted, and the diff revision is greater than one.

Manually tested all new features and examined resulting emails.

For item 1, tested review requests both with and without diffs.
For item 2, tested review request with multiple diff revisions and verified that View Diff link corresponded to the appropriate specific revision number.
For item 3, tested that Show Changes link did not appear for diff revision 1, but did appear for subsequent revisions and had the correct interdiff range.